Diagenetic transformation of dolomite into stevensite in lacustrine sediments from Jbel Rhassoul, Morocco

Stevensite in the Tertiary lacustrine basin of the Jbel Rhassoul in Morocco is used as fuller's earth. Observations of the structure and textures of t he deposits provide evidence of diagenetic replacement of sedimentary lacus trine dolomite by stevensite. Carbon and oxygen isotopic compositions confi rm the precipitation of primary sedimentary dolomite and the diagenetic for mation of secondary stevensite, According to thermodynamic calculations, st evensite is stable at the expense of dolomite in a silica-rich and CO2-poor environment. Under such conditions, destabilized detrital illite, chlorite , and palygorskite release Al, Fe, Li, and trace elements, which are incorp orated into the structure of stevensite, Compared with other lacustrine ste vensite occurrences, the Jbel Rhassoul deposit seems to correspond to a new mode for the formation of diagenetic stevensite after dolomite.